22,363 research outputs found
Anomalous spin-dependent behaviour of one-dimensional subbands
We report a new electron interaction effect in GaAs/AlGaAs quantum wires.
Using DC-bias spectroscopy, we show that large and abrupt changes occur to the
energies of spin-down (lower energy) states as they populate. The effect is not
observed for spin-up energy states. At B=0, interactions have a pronounced
effect, in the form of the well-known 0.7 Structure. However, our new results
show that interactions strongly affect the energy spectrum at all magnetic
fields, from 0 to 16T, not just in the vicinity of the 0.7 Structure.Comment: 4 pages, 2 figure
Small gaps between products of two primes
Let denote the number that is a product of exactly two
distinct primes. We prove that
This sharpens an earlier result of the authors (arXivMath NT/0506067), which
had 26 in place of 6. More generally, we prove that if is any positive
integer, then
We also prove several other results on the representation of numbers with
exactly two prime factors by linear forms.Comment: 11N25 (primary) 11N36 (secondary
Cancellation exponent and multifractal structure in two-dimensional magnetohydrodynamics: direct numerical simulations and Lagrangian averaged modeling
We present direct numerical simulations and Lagrangian averaged (also known
as alpha-model) simulations of forced and free decaying magnetohydrodynamic
turbulence in two dimensions. The statistics of sign cancellations of the
current at small scales is studied using both the cancellation exponent and the
fractal dimension of the structures. The alpha-model is found to have the same
scaling behavior between positive and negative contributions as the direct
numerical simulations. The alpha-model is also able to reproduce the time
evolution of these quantities in free decaying turbulence. At large Reynolds
numbers, an independence of the cancellation exponent with the Reynolds numbers
is observed.Comment: Finite size box effects have been taken into account in the
definition of the partition function. This has resulted in a more clear
scaling in all figures. Several points are clarified in the tex
Energy-level pinning and the 0.7 spin state in one dimension: GaAs quantum wires studied using finite-bias spectroscopy
We study the effects of electron-electron interactions on the energy levels
of GaAs quantum wires (QWs) using finite-bias spectroscopy. We probe the energy
spectrum at zero magnetic field, and at crossings of opposite-spin-levels in
high in-plane magnetic field B. Our results constitute direct evidence that
spin-up (higher energy) levels pin to the chemical potential as they populate.
We also show that spin-up and spin-down levels abruptly rearrange at the
crossing in a manner resembling the magnetic phase transitions predicted to
occur at crossings of Landau levels. This rearranging and pinning of subbands
provides a phenomenological explanation for the 0.7 structure, a
one-dimensional (1D) nanomagnetic state, and its high-B variants.Comment: 6 pages, 4 figure
Quantum states on supersymmetric minisuperspace with a cosmological constant
Spatially homogeneous models in quantum supergravity with a nonvanishing
cosmological constant are studied. A class of exact nontrivial solutions of the
supersymmetry and Lorentz constraints is obtained in terms of the Chern-Simons
action on the spatially homogeneous 3-manifold, both in Ashketar variables
where the solution is explicit up to reality conditions, and, more concretely,
in the tetrad-representation, where the solutions are given as integral
representations differing only by the contours of integration. In the limit of
a vanishing cosmological constant earlier exact solutions for Bianchi type IX
models in the tetrad-representation are recovered and additional asymmetric
solutions are found.Comment: 14 pages, late
Dynamical preparation of EPR entanglement in two-well Bose-Einstein condensates
We propose to generate Einstein-Podolsky-Rosen (EPR) entanglement between
groups of atoms in a two-well Bose-Einstein condensate using a dynamical
process similar to that employed in quantum optics. The local nonlinear S-wave
scattering interaction has the effect of creating a spin squeezing at each
well, while the tunneling, analogous to a beam splitter in optics, introduces
an interference between these fields that results in an inter-well
entanglement. We consider two internal modes at each well, so that the
entanglement can be detected by measuring a reduction in the variances of the
sums of local Schwinger spin observables. As is typical of continuous variable
(CV) entanglement, the entanglement is predicted to increase with atom number,
and becomes sufficiently strong at higher numbers of atoms that the EPR paradox
and steering non-locality can be realized. The entanglement is predicted using
an analytical approach and, for larger atom numbers, stochastic simulations
based on truncated Wigner function. We find generally that strong tunnelling is
favourable, and that entanglement persists and is even enhanced in the presence
of realistic nonlinear losses.Comment: 15 pages, 19 figure
Feedback-enhanced algorithm for aberration correction of holographic atom traps
We show that a phase-only spatial light modulator can be used to generate
non-trivial light distributions suitable for trapping ultracold atoms, when the
hologram calculation is included within a simple and robust feedback loop that
corrects for imperfect device response and optical aberrations. This correction
reduces the discrepancy between target and experimental light distribution to
the level of a few percent (RMS error). We prove the generality of this
algorithm by applying it to a variety of target light distributions of
relevance for cold atomic physics.Comment: 5 pages, 4 figure
Virtual Data in CMS Analysis
The use of virtual data for enhancing the collaboration between large groups
of scientists is explored in several ways:
- by defining ``virtual'' parameter spaces which can be searched and shared
in an organized way by a collaboration of scientists in the course of their
analysis;
- by providing a mechanism to log the provenance of results and the ability
to trace them back to the various stages in the analysis of real or simulated
data;
- by creating ``check points'' in the course of an analysis to permit
collaborators to explore their own analysis branches by refining selections,
improving the signal to background ratio, varying the estimation of parameters,
etc.;
- by facilitating the audit of an analysis and the reproduction of its
results by a different group, or in a peer review context.
We describe a prototype for the analysis of data from the CMS experiment
based on the virtual data system Chimera and the object-oriented data analysis
framework ROOT. The Chimera system is used to chain together several steps in
the analysis process including the Monte Carlo generation of data, the
simulation of detector response, the reconstruction of physics objects and
their subsequent analysis, histogramming and visualization using the ROOT
framework.Comment: Talk from the 2003 Computing in High Energy and Nuclear Physics
(CHEP03), La Jolla, Ca, USA, March 2003, 9 pages, LaTeX, 7 eps figures. PSN
TUAT010. V2 - references adde
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